WPF ComboBox with a title in the popup

MainWindow.xaml

<Window x:Class="WpfApp1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Window.Resources>
        <Style TargetType="ComboBox">
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="ComboBox">
                        <Grid>
                            <ToggleButton x:Name="DropDownToggle"
                                          HorizontalAlignment="Stretch" VerticalAlignment="Stretch"
                                          Margin="-1" HorizontalContentAlignment="Right"
                                          IsChecked="{Binding Path=IsDropDownOpen,Mode=TwoWay,
                                RelativeSource={RelativeSource TemplatedParent}}">
                                <Path x:Name="BtnArrow" Height="4" Width="8"
                                      Stretch="Uniform" Margin="0,0,6,0" Fill="Black"
                                      Data="F1 M 300,-190L 310,-190L 305,-183L 301,-190 Z " />
                            </ToggleButton>
                            <ContentPresenter x:Name="ContentPresenter" Margin="6,2,25,2"
                                              Content="{TemplateBinding SelectionBoxItem}"
                                              ContentTemplate="{TemplateBinding SelectionBoxItemTemplate}"
                                              ContentTemplateSelector="{TemplateBinding ItemTemplateSelector}" />
                            <TextBox x:Name="PART_EditableTextBox"
                                     Style="{x:Null}"
                                     Focusable="False"
                                     Background="{TemplateBinding Background}"
                                     HorizontalAlignment="Left"
                                     VerticalAlignment="Center"
                                     Margin="3,3,23,3"
                                     Visibility="Hidden"
                                     IsReadOnly="{TemplateBinding IsReadOnly}" />

                            <Popup x:Name="PART_Popup"
                                   IsOpen="{TemplateBinding IsDropDownOpen}">
                                <Border x:Name="PopupBorder"
                                        HorizontalAlignment="Stretch" Height="Auto"
                                        MinWidth="{TemplateBinding ActualWidth}"
                                        MaxHeight="{TemplateBinding MaxDropDownHeight}"
                                        BorderThickness="{TemplateBinding BorderThickness}"
                                        BorderBrush="Black" Background="White" CornerRadius="3">
                                    <StackPanel>
                                        <Label>My title here</Label>
                                        <ScrollViewer x:Name="ScrollViewer" BorderThickness="0" Padding="1">
                                            <ItemsPresenter />
                                        </ScrollViewer>
                                    </StackPanel>
                                </Border>
                            </Popup>

                        </Grid>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </Window.Resources>

    <Grid>
        <ComboBox Height="26" Width="150">
            <ComboBoxItem Name="Item1">Item1</ComboBoxItem>
            <ComboBoxItem Name="Item2">Item2</ComboBoxItem>
            <ComboBoxItem Name="Item3">Item3</ComboBoxItem>
            <ComboBoxItem Name="Item4">Item4</ComboBoxItem>
        </ComboBox>
    </Grid>
</Window>